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This invention relates to a control system for controlling the electrical current supplied to a solenoid, such as the solenoid of a solenoid-operated hydraulic control valve.
It is known to use a pulse-width modulation technique to control the current supply to the solenoid coils of electrohydraulic valves. This is a voltage control technique, but because solenoid forces are proportional to solenoid current, a method of compensating for variations in supply voltage and coil resistance is required, for example, an operator-adjustable sensitivity control. Another known method of controlling solenoid current is to provide a current feedback sensor in series with the coil. This current is sensed by means of a comparator which turns the driver off when the coil current exceeds a reference level and turns the driver back on when the current drops below the reference level. The hysteresis of the comparator circuit provides a controllable off and on time for this circuit. This valve driver method will provide an accurate control of the average solenoid current for both varying supply voltages and varying coil resistances. However, the cost and number of electronic components to achieve this method can be significantly greater than the pulse width modulation method. Accordingly, a low cost and effective system for controlling solenoid current is desired.